The present invention relates to a cooking appliance that automatically controls the operation of the cooking appliance by detecting the surface temperature of food by a sensor such as a thermistor.
The following example refers to an oven toaster. In order to correctly identify the condition of food being cooked, an oven toaster using a quartz-tube electric heating device needs to constantly monitor the condition of the food being cooked by a sensor. It is required that a system that controls the operation of the cooking appliance by detecting the temperature of cooking food detect even the slightest thermal energy radiated from the surface of food cooking. To achieve this, the requirements listed below have to be taken into account when installing the sensor. (1) The sensor should be located in a position as close to the food as possible; (2) The distance between the food and sensor should be constant; and (3) The sensor should not encounter extremely high temperatures, i.e., the sensor should remain perfectly free from this influence of heat sources such as the heating device.
Conventionally, a cooking appliance is provided with a sensor 11 in a position above the food to be cooked, as shown in FIG. 1. Since the thermal energy generated by a heater 3 for cooking food 4 rises upward, the sensor 11 in unavoidably exposed to high temperatures and thus influenced by the heated generated by the heater 3 that remains inside the chamber 14 of a cooking appliance 10. To keep the distance 17 between the sensor 11 and food 4 constant, it is necessary to vary the position of sensor 11 in accordance with the thickness of the food 12.
FIG. 2 describes the relationship between the voltage output from the sensor and the ambient temperature. The vertical axis indicates that detection signal of voltages output from the sensor, whereas the horizontal axis denotes temperatures surrounding the sensor. Despite the constant temperature of the heat source and the constant distance between the food and the heat source, the voltage output from the sensor decreases significantly as the temperature surrounding the sensor rises. Consequently, when a cooking appliance such as that shown in FIG. 1 is used, as the temperature in the cooking chamber of said appliance 10 rises by the heater 3, the surrounding temperature of the sensor 11 and the temperature of sensor 11 itself also rises. Therefore, the output from the sensor 11 when the sensor 11 detects the thermal energy generated from cooking food 4 decreases to an extremely low level, as shown in FIG. 1. Consequently, to amplify the detecting signal to the desired level, a number of amplifying circuits are needed. However, when the output signal of the sensor is amplified, a variety of errors present in the detection signal are also amplified. Conventional cooking appliances cannot therefore generate the correct output signal needed to accurately detect the thermal energy radiating from food being cooked.